JPH0421849B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical modulator that uses the electro-optic effect of a dielectric crystal, and in particular to an optical modulator that has an optical waveguide formed on the surface of a crystal substrate, has a low operating voltage, high operating speed, and has a simple structure. Related to waveform light modulators.

The optical light modulator has long been researched and developed as a constrained light modulator by utilizing the result of a change in refractive index proportional to the intensity of an electric field applied to a crystal (Pockels effect). A light beam is transmitted through a large-sized crystal, and an electric field is applied in a direction perpendicular to the direction of transmission of the light beam to rotate the plane of polarization of the light beam transmitted through the crystal. Bulk type optical modulators based on the principle of changing light intensity by transmitting light have the disadvantages of high applied voltage and lack of temperature stability. One of the causes of high applied voltage is that when the light transmission length of the crystal is increased and the distance between the electrodes is narrowed, that is, the thickness of the crystal is thinned, light diffraction occurs at the light input/output surface, so the length of the crystal increases. This is due to the fact that the ratio to the crystal thickness cannot be made larger than a certain value. In order to prevent light diffraction from occurring even if the crystal length is increased, it is necessary to have a waveguide structure.

A layer or channel with a high refractive index is provided on the surface of a dielectric or semiconductor substrate, and this is used as an optical waveguide.A voltage is applied between opposing electrodes placed near this optical waveguide, and the generated voltage is generated in the optical waveguide. The refractive index in the waveguide is changed by the electric field generated by the waveguide, causing optical phenomena such as diffraction, refraction, scattering, and mode conversion in the guided light that travels there, and modulation is performed by changing the transmission amplitude of the guided light. The device is called a waveguide type optical modulator. The advantage of waveguide optical modulators is that the length of the electric field, that is, the length of the element, can be increased due to the small diffraction loss of light caused by waveguide, as described above, and the fact that the length of the waveguide layer or waveguide is small. Another advantage of this method is that it has high sensitivity because the electrodes are closely spaced and the applied electric field strength can be increased.
One of the conventionally known methods of constructing a specific element is the Bragg diffraction type, in which a layer with a high refractive index is provided uniformly on the surface of a crystal plate that has an electro-optic effect, and this is used as an optical waveguide. layer, further providing interdigital electrodes on this waveguide layer,
A periodic electric field formed in the waveguide layer by the electric field applied to this electrode causes the guided light to be diffracted within the substrate plane,
It modulates the intensity of transmitted light. This type of optical modulator has a flat waveguide, so in order to use it for modulating fiber light, it is necessary to couple the light with the optical fiber by using a cylindrical lens or the like at the input/output end of the waveguide. However, the structure becomes complicated, and there are concerns about increased optical loss, reliability, and stability.

Another example of a waveguide type optical modulator is a device called a directional coupling type, and the principle of this device is that two channel waveguides are provided very close to each other on the surface of an electro-optic crystal plate. Light is incident on one waveguide, and the amount of light coupled to the other waveguide in a region close to one waveguide is controlled by an electric field generated from an electrode provided on the surface of the crystal plate. Since the waveguide of this type of element is in the form of a channel, it has the advantage of being easy to couple with an optical fiber or the like. The method of coupling light is to set up an element so that most of the light moves from one waveguide to the other without applying an electric field, and then apply an electric field to change the refractive index of both waveguides. A method of breaking the coupling by changing the phase constant is used because it requires only a low applied voltage. However, in order to reduce optical loss, it is necessary that complete coupling occurs without an electric field being applied. Requires precision. In reality, it is difficult to create such a state, so by devising the electrode structure,
It is designed so that either light coupling or dissociation occurs at two voltage values. Therefore, when used as an optical pulse modulator, a bias potential is required. Furthermore, the characteristics vary greatly when the wavelength of the transmitted light changes, and it is necessary to optimally design the element according to the wavelength. In addition, the width and spacing between two parallel waveguides is minute, 3 to 10 μm, and the length is 10 to 20 mm.
and long. It is very difficult to obtain such a pattern without defects and at a high yield, and the manufacturing cost is high.

Another example of a waveguide type optical modulator is a modulator that converts guided light traveling through a waveguide layer into a radiation mode in a substrate by applying an electric field. Specifically, metal ions are diffused on the surface of a C-cut plate made of lithium niobate crystal, which is a birefringent crystal, and a high refractive index layer with a slightly higher refractive index than that of the substrate is provided.
A TM wave, which is a light wave mode having an oscillating electric field component in the C-axis direction, is excited. When an electric field is applied within the substrate plane in a direction perpendicular to the light transmission direction, the TM wave is converted into a radiation mode that has an oscillating electric field component parallel to the substrate and radiates into the substrate, reducing the intensity of the guided TM mode. do. Waveguide optical modulators based on this type of configuration have the advantage of simple configuration, but waveguide
Since there is a large refractive index difference between the refractive index of the TM mode and the radiation mode having an oscillating electric field component parallel to the substrate, the conversion efficiency is low and a high applied voltage is required, making it less practical.

As described above, all conventional waveguide optical modulators have drawbacks. An object of the present invention is to provide a high-performance, inexpensive waveguide optical modulator that eliminates the above-mentioned drawbacks.

According to the present invention, there is provided a channel-shaped optical waveguide formed near the surface of an electro-optic crystal, and ions provided around the optical waveguide, the thickness of which changes periodically along the light transmission direction of the optical waveguide. By using the exchange layer and the means for applying an electric field into the channel-type optical waveguide, a waveguide type electro-optic optical modulator that is high-performance, inexpensive, and has excellent temperature characteristics can be obtained.

FIG. 1 is a diagram explaining the principle on which the waveguide electro-optic optical modulator of the present invention relies. 1 is a carbon lithium niobate crystal plate, 2 is a channel optical waveguide made of thermally diffused titanium, and 3 is a channel optical waveguide. A layer 4 subjected to ion exchange treatment near the substrate surface other than the waveguide 2 is a metal electrode film that covers the area of the layer 3 subjected to ion exchange treatment, and 5 is a channel optical waveguide 2.
The linearly polarized light 6 parallel to the substrate that is incident on the substrate has a potential applied to the electrodes 4 facing each other across the channel optical waveguide. When a voltage is applied to the application electrode 4, the light incident on the channel optical waveguide 2 is
As it propagates through the optical waveguide 2, it is converted into emitted light that spreads to the ion exchange layer 3, and this emitted light is attenuated by the electrodes 4 and disappears. When no voltage is applied to the application electrode 4, the incident light 5 is guided through the channel waveguide 2 and exits without being attenuated. The intensity of the light guided through the channel waveguide 2 and emitted varies depending on the magnitude of the applied voltage 6, and the intensity of the light is modulated. This operation is explained by the mechanism described below.

Lithium niobate crystal has uniaxial optical anisotropy, for example, the refractive index for light waves with a wavelength of 1.3 μm is the refractive index for light waves having an oscillating electric field component in the C-axis direction, that is, the extraordinary light refractive index n _e is 2.145, C The refractive index, n _p , for a light wave having an oscillating electric field component in a direction perpendicular to the axis has a value of about 2.222.
When a titanium metal film is provided on the surface of this crystal by vapor deposition or the like and exposed to a high temperature of about 1000°C, the titanium metal diffuses into the crystal, creating a region with a slightly higher refractive index near the substrate surface. The increase in refractive index is 5×
It is about 10 ^-3 . When a thin titanium film is provided in the form of a thin line and thermally diffused, the channel waveguide 2 shown in Figure 1 is formed.
can be generated. The equivalent refractive index of the waveguide mode propagating through this channel waveguide, that is, the value obtained by dividing the propagation wave number by the wave number in air (equivalent refractive index), differs depending on the cross-sectional size of the channel waveguide. ) has a value between 2.225 and 2.222, and that of the mode of polarization perpendicular to the substrate (TM mode) has a value between 2.150 and 2.145.
has a value between . Furthermore, when the lithium atom in the lithium niobate crystal is replaced with another atom, such as an atom such as silver or hydrogen, only the extraordinary refractive index n _e increases, and the ordinary refractive index n _p does not change. For example, if lithium niobate crystals are boiled in benzoic acid at a temperature of about 249℃ for 1 hour and subjected to ion exchange treatment, 2μ
Over a depth of about m, lithium ions in the crystal are replaced by hydrogen atoms, and n _e is about 0.11 (λ = 1.3 μ
m) rise; The ion exchange layer 3 in Figure 1 serves as a good waveguide for the TM mode, and its equivalent refractive index is 2.255, which varies depending on the thickness of the ion exchange layer.
has a value between ~2.145. This ion exchange does not occur if it is covered with a metal film during exchange, and by selectively covering the surface of the titanium diffusion waveguide portion shown in Figure 1 with a thin metal film during exchange, it is possible to remove ions other than channel waveguide 2. Ion exchange only near the surface is achieved.

When an electric field is applied in the x direction of a lithium niobate crystal, coupling occurs between the ordinary light and the extraordinary light through the electro-optic constant _r51 , but for efficient conversion to occur, the refractive indices must be approximately the same. must be maintained. As shown in the configuration of the embodiment in FIG. 1, a metal electrode 4 is provided on the ion exchange waveguide 3, and a voltage of 6
When the TE wave 5 is applied to the channel waveguide 2, the x generated in the channel waveguide 2
The applied electric field in the direction converts the TE wave into a radiation TM mode that dissipates in the xy plane in the ion exchange waveguide. This is because TE propagating through the channel waveguide
The equivalent refractive index of the wave n _g can be set to a value between 2.225 and 2.222 as described above, and the equivalent refractive index n _r of the ion exchange layer can also be set to a value between 2.255 and 2.145 as described above. I can do it. The equivalent refractive index n _r of the ion exchange layer is set to be slightly larger than the equivalent refractive index n _g of the channel waveguide, for example, by about 1×10 ⁻⁴ . A diagram like the one shown in FIG. 2 makes it possible to understand the matching relationship between the channel guided light and the emitted light to the ion exchange layer. Radiation TM of ion exchange layer
Since the equivalent refractive index of guided light is the same in the xy plane,
It can be expressed as a circle with radius n _r . The wavefront traveling direction of the channel guided light is the g direction, and the y
It is expressed as a vector with length n _g in the direction. The channel guided light is converted into in-plane radiated TM guided light of the ion exchange layer in the direction of the angle θ that satisfies the relationship cosθ=n _g /n _r (1) by applying an electric field. For the above equivalent refractive index difference setting, that is, n _r −n _g =1×10 ⁻⁴ , radiation is emitted in the θ0.5° direction. As is well known,
When the waveguide layer surface is covered with a metal film, the attenuation of the TM wave is extremely large, close to 100 dB/cm. Therefore, the radiation TM waveguide light of the ion exchange layer is channel guided.
When converted from TE light, it is immediately absorbed by the metal film.

However, in this configuration, since the temperature coefficient of birefringence of the lithium niobate crystal is large, the optical modulation characteristics change greatly with temperature, making it unusable.
The temperature coefficient of birefringence of lithium niobate crystals that has already been reported is |d( _ne −n _p )/d _T |=4:3×10 ⁻⁵ (1). For example, if the ambient temperature changes by ±25°C, the amount of change in birefringence will reach ±1×10 ^-3 . The equivalent refractive index of the guided mode changes by almost the same amount as the refractive index of the substrate changes. As mentioned above, the difference between the equivalent refractive index n _g of the channel-guided TE mode and the equivalent refractive index n _r of the in-plane radiation TM mode of the ion exchange layer is set to about 1 × 10 ^-4 . If the temperature changes by 3°C, a state where n _g > n _r will appear. In this case, in the diagram showing the matching relationship between the two equivalent refractive indexes in Figure 2, the tip of the arrow indicating n _g will go outside the semicircle with radius n _r , making it impossible to match. . Even if an electric field is applied, it becomes impossible to cause mode conversion, that is, modulation of the intensity of channel guided light.

In order to avoid this, a large difference in equivalent refractive index is given in advance, and the setting is made so that n _g <n _r always holds in the diagram in FIG. 2 even if the temperature changes. For example, if n _r −n _g 2×10 ^-5 , ±
Within a temperature change of 25° C., the condition n _g <n _r is always satisfied, and there is an angle at which the guided light propagating through the channel waveguide matches the in-plane radiation mode in the ion exchange layer. However, the efficiency of conversion from guided mode to radiation mode varies greatly depending on temperature.
The above conversion efficiency becomes lower as the radiation angle becomes larger. This is because the intensity distribution of the optical electric field in the radiation mode with a large radiation angle is significantly different from the intensity distribution of the optical electric field in the waveguide mode. A temperature change occurs in which the conversion efficiency is high at temperatures where the magnitudes of the phase constants of the two modes become close, and the efficiency decreases at temperatures where the difference becomes large.

In order to prevent the efficiency of conversion from guided mode to radiation mode from changing due to changes in temperature, measures are taken to ensure that the radiation angle to radiation mode does not change even if the magnitude of birefringence changes. Just do it. FIG. 3 is a diagram showing the structure of an embodiment of the present invention, in which 1 is a Z-cut lithium niobate crystal plate, 2 is a titanium diffusion channel waveguide, 3 is an ion exchange layer, and the upper surface of the ion exchange 3 is a channel waveguide. 2 is covered by an electrode 4 for applying an electric field. 5 is an incident TE wave, and 6 is a voltage source applied to the electrode 4.
The depth of the ion exchange layer 3 changes periodically along the light transmission direction of the channel waveguide 2,
The period changes monotonically from Λ ₁ to Λ ₂ (Λ ₂ >Λ ₁ ). FIG. 4 shows a cross section of the lithium niobate crystal shown in FIG. 3 taken perpendicularly to the x-axis. changes periodically in the y direction, and the period is from Λ ₁ to Λ ₂
It shows a structure that changes monotonically in the y direction until .

The matching relationship between guided mode and radiation mode can be understood from FIG. From the equivalent refractive index n _g of the guided mode,
Average thickness d of the ion exchange layer shown in Figure 4
The thickness d is determined so that the equivalent refractive index n _r of the radiation mode at is small. The equivalent refractive index of the waveguide mode is indicated by the arrow pointing in the y direction, and the radiation mode is
It can be expressed as a semicircle with radius n _r in the xy plane. Since the thickness of the ion exchange layer changes periodically in the y direction, the equivalent refractive index at which a radiation mode can exist due to this spatial lattice is n _r + λ/Λ ₂ to n _r +
Continuously distributed up to λ/Λ ₁ . Here, λ is the wavelength of light. Since the spatial grating spectrum is oriented in the y direction, in the xy plane, as shown in FIG. 5, the area indicated by the crescent-shaped diagonal line is an area where a radiation mode can exist. Equivalent refractive index of guided mode
Set so that n _g is located within the crescent-shaped hatched area in FIG. A temperature change in birefringence is a change in the difference between the equivalent refractive index of the guided mode and the equivalent refractive index of the radiation mode. If the change is ±△n _T , then the waveguide mode at room temperature is as follows: n _g +△n _T <n _r +λ/Λ ₁ (2) n _g −△n _T >n _r +λ/Λ ₂ (3) Determine the refractive index n _g . With this setting, n _g in Figure 5
The destination of the vector always lies within the shaded area. Therefore, unlike the case where the equivalent refractive index of the radiation mode is represented by a single arc in the xy plane when the thickness of the ion exchange layer is uniform, as shown in Figure 2, when the temperature changes, The conversion efficiency does not change as the radiation angle changes.

In the above case, the following values can be set as specific numerical values. When the light wavelength is 1.3μm,
The average thickness d of the ion exchange layer is determined so that the equivalent refractive index n _g of the waveguide mode is 2223 and the equivalent refractive index n _r of the radiation mode is 2220. Temperature is ±25
As mentioned above, the amount of change in birefringence when the temperature changes is △n _T
1×10 ^-3 , so from equations (2) and (3) above, Λ ₁
= 325 μm, Λ ₂ = 650 μm. In other words, the period
The unevenness of the ion exchange thickness may be provided by changing the thickness almost continuously from 325 μm to 625 μm. Changes in the thickness of the ion-exchange layer can be achieved by providing an aluminum vapor-deposited film or the like in a lattice pattern, boiling it in benzoic acid, and then performing the same ion-exchange treatment after removing the aluminum film. The thickness can be controlled by controlling the exchange processing time.

Equivalent refractive index n _r of the radiation TM mode of the ion exchange layer
may be set to be larger than the equivalent refractive index n _g of the guided TE mode. That is, even if n _g +△n _T ＜n _r −λ／Λ ₂ (4) n _g −△n _T ＞n _r −λ／Λ ₁ (5), the waveguide is It always exists within the shaded area beyond the vector of n _g indicating the equivalent refractive index of the mode, and temperature changes in the conversion efficiency from the waveguide mode to the radiation mode are also suppressed. In this case, the specific values are λ=1.3μm, n _g
= 2.223, n _r = 2.226, Λ ₁ = 325 μm, and Λ ₂ = 650 μm.

In the above explanation, the case of a straight channel waveguide has been described. Of course, if a waveguide is curved in the substrate plane, for example, if the polarity radius is set to have a small radiation loss for guided TE waves, then in the curved part, the intensity distribution of the guided light will be biased to the outside of the curve within the channel, resulting in a radiation mode. This makes it easier to couple with the material, and requires less applied voltage.

As explained above, the waveguide optical modulator of the present invention has the following advantages compared to conventionally known waveguide optical modulators:
Since it is composed of a single channel waveguide, the modulation characteristics do not depend greatly on the device fabrication precision, and no bias voltage is required, it is superior to the directionally coupled modulator described above, and can be used for radiation modes within the substrate plane. Since the applied voltage characteristics are not limited by the inherent birefringence of the substrate, and the synchrotron radiation can be absorbed in-plane, the modulation characteristics will not deteriorate due to light reflected from the back surface of the substrate. This is superior to the substrate radiation type optical modulation described above. And its operation is extremely stable against temperature changes.

In the above embodiment, a case where a lithium niobate crystal plate is used as a substrate has been described. Even if other electro-optic crystals such as lithium tantalate crystals are used, the ion exchange layer can be formed in the same way, and the device can be constructed in the same way.

In addition, we have described a case in which a C-cut (Z-cut) plate of an electro-optic crystal is used to convert channel guided light into synchrotron radiation using an applied electric field along the crystal substrate. Alternatively, a method using an electric field perpendicular to the substrate surface can be used.

In addition, as a method for forming channel waveguides, we have described the case where metal is thermally diffused into a substrate, but an epitaxially grown layer of electro-optic crystal that is lattice-matched to the substrate is used, and a rib-shaped waveguide or a rib-shaped waveguide is formed in this grown layer. , a waveguide or the like whose surface is loaded with a dielectric material may be formed.

As described above, according to the present invention, an inexpensive and high-performance waveguide optical modulator can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the principle on which the present invention is based.
1 is an electro-optic crystal, 2 is a metal ion diffusion waveguide,
3 is an ion exchange layer, 4 is an electrode, and 5 is incident light. FIG. 2 is a diagram illustrating the matching conditions between the channel guided light and the emitted light to explain the above principle. FIG. 3 is a diagram showing the structure of an embodiment of the present invention, in which 1 is an electro-optic crystal, 2 is a metal ion diffusion waveguide, 3 is an ion exchange layer, 4 is an electrode, 5 is an incident light beam,
6 is a voltage source. FIG. 4 is a sectional view perpendicular to the x-axis of FIG. 3. FIGS. 5 and 6 are diagrams showing matching conditions between the channel guided light and the emitted light to explain the operation of the embodiment of the present invention shown in FIG.

Claims (1)

[Claims] 1. A waveguide type electric waveguide comprising a single channel type optical waveguide formed near the surface of an electro-optic crystal, an ion exchange layer provided around the optical waveguide, and means for applying an electric field into the channel type optical waveguide. 1. A waveguide electro-optic modulator, characterized in that the thickness of the ion exchange layer changes periodically along the light transmission direction of the channel-type optical waveguide.